1. Field of the Invention
The present invention relates to altimeters, and in particular, to accumulating altimeters which selectively accumulate total altitude gains or losses for ascents or descents, respectively, for a selected period of time.
2. Description of the Related Art
With the development of solid state altimeters, such as those using piezoelectric or piezoresistive elements, use of altimeters for various activities has grown. One such use is for bicyclists to record altitude gained or lost during a bicycle trip. In particular, those who train and compete in bicycle races often wish to know the altitude gain or loss during a training ride. This helps provide an insight to the training regimen and whether it is correct for the event for which they are training.
Conventional accumulating altimeters can provide information regarding altitude gain or loss. For example, U.S. Pat. No. 4,694,694 discloses an altimeter which accumulates and displays total altitude gains or losses over a selected interval of time. If set to accumulate and display total gains in altitude, any and all altitude gains are accumulated and displayed.
However, such indiscriminate altitude accumulation may not be meaningful and can be misleading. During virtually any bicycle ride, slight undulations in terrain, including grade separations at railroads and highways, as well as transient barometric pressure variations can cause the altimeter to mistakenly detect that it has gained or lost altitude. This causes the accumulated altitude (gain or loss) to increase, although no net change has occurred. In other words, even though the instrument may have detected a number of altitude changes while traversing a number of rises or depressions in the roadway, such changes do not correctly represent the work or effort put forth by the rider.
For a cyclist, ascents and descents of grade separations are of no interest. If the accumulating altimeter accumulates all altitude gain or loss, however small, the cyclist will not receive meaningful information regarding the effort put forth, and therefore the training value of the ride.
Furthermore, the altimeter is subject to mechanical vibration and electrical noise within its own circuits. These also can cause the altimeter to accumulate further altitude gains or losses. The errors introduced from these effects can be substantial. For example, typical altitude measurement errors can be on the order of 10-20 feet due to mechanical vibration or electrical noise within the altimeter.
Therefore, it is desirable to have an accumulating altimeter which can selectively accumulate altitude changes such as gains, while selectively ignoring some, but not all, opposing altitude changes such as losses. In other words, it would be desirable to have an accumulating altimeter that can selectively ignore insignificant altitude changes, due to such things as mechanical vibration, slightly rolling terrain or highway overpasses, while at the same time being capable of recognizing significant altitude changes, due to such things as riding over a hill or into a valley.